This invention relates to a starting circuit for a chopper-powered, thyristor inverter-motor system, where the thyristors in the inverter are motor-commutated during running operation.
Anytime a motor-commutated thyristor inventer is employed to drive a motor, a starting system is required to initiate motor rotation and increase the speed until sufficient commutating motor voltage is provided. To explain, the thyristor switching devices (which usually take the form of silicon controlled rectifiers or SCR's) in the inverter are gated or turned on in predetermined sets and in a prescribed sequence in order to convert an applied d-c bus voltage, received over a d-c bus from a d-c power supply, to a-c voltage for application to the motor. Before each set of thyristors are fired into conduction, at least one previously conducting thyristor must be commutated or switched off. Forced commutation circuitry may be obviated by using a motor, such as a synchronous motor, which is constructed to present a leading power factor to the inverter drive, the alternating current in each of the motor's stator windings thereby always leading the alternating motor voltage across that winding. Design techniques for constructing a synchronous motor to have a leading power factor are well understood in the art. Basically, it involves providing a motor back EMF (electromotive force) that is greater than the applied inverter voltage. The back EMF is induced in the stator windings by the rotating flux produced by the magnet (either a permanent magnet or an electromagnet) in the rotor. With a leading power factor the thyristors will be motor-commutated, meaning that when a thyristor is gated on it will cause the back EMF to reverse bias and to turn off a previously conducting thyristor, the motor current thereby effectively transferring to the on-comming thyristor.
The problem presented with a motor-commutated thyristor inverter is that the motor must be running in order for the rotating flux from the rotor to cut the stator windings and induce therein a back EMF of adequate magnitude to commutate the thyristors. A starting system of some type must therefore be employed to start the motor rotating and bring it up to a speed at which the required back EMF will develop and take over the commutation of the inverter. Starting systems have been developed which regulate the d-c power supply and the inverter to apply time-separated current pulses to the stator windings of the motor to effect step-by-step motor rotation at an increasing or accelerating rate until the back EMF reaches a minimmum threshold level at which motor commutating and normal running operation occur.
When the d-c power supply comprises a phase-controlled rectifier bridge, which rectifies a-c line voltage and produces an adjustable d-c bus voltage, starting of a motor-commutated thyristor inverter may be achieved by turning the thyristors on in predetermined sets and in a prescribed sequence to current pulse energize the motor windings, which produces torque to crank or turn the motor one step at a time. During the starting mode, before each set of thyristors are gated on, the previously conducting thryistors must be commutated off by some means. When a current pulse is supplied from the phase-controlled rectifier bridge and through the conducting thyristors to the motor windings to turn the motor one step, reactive energy builds up and becomes stored in the inductances through which that motor current passes. In addition to the motor inductance, the motor current generally flows through a separate inductance connected in series with the d-c link or bus to provide current limiting protection in the event of a short circuit across the d-c bus and to filter or smooth out the current. The reactive energy locked in these inductances at the termination of each current pulse delivered to the motor must, of course, be removed before the conducting thyristor set can be commutated off and the next set turned on. The necessary removal of energy has been accomplished by regenerating energy from the load and back into the a-c line. The reactive power is made to flow from the inductances to the a-c line voltage, where the power is rapidly absorbed. Supplying the reactive energy through the phase-controlled rectifier bridge to the a-c line is achieved by appropriate gating or phasing of the rectifying devices (usually SCR's) in the rectifier bridge. Relatively fast absorption of the reactive energy is necessary to minimize the decay time of the motor current to zero at the termination of each energizing pulse so that the next set of thyristors may be switched on to turn the motor another step. Of course, the pulse actuation of the motor must eventually occur at a rate fast enough to allow the back EMF to develop and take over the commutating function, the inverter-motor system thereby becoming self-commutating. Hence, the decay time of the motor must be short enough so that the energizing current pulses may be supplied to the motor at the frequency needed to develop an adequate back EMF commutating voltage in the motor.
If the motor-commutated inverter is controlled by a chopper, rather than a phase-controlled rectifier bridge, motor starting becomes much more difficult. A chopper is a D-C to D-C converter having a chopper switch which is alternated between on and off conditions at a relatively high frequency, such as around 2,000 cycles per second or hertz, to pulse-width modulate or duty cycle modulate a fixed magnitude d-c voltage, usually developed from a-c line voltage by a full wave rectifier bridge, to produce chopped d-c voltage, namely a series of periodically recurring voltage pulses of adjustable width. The chopper is followed by a relatively small inductor, connected in series with the d-c bus or link and customarily called a delay inductance, whose prime purpose is to provide current limiting in the event of a lock-on of two SCR's in the inverter. The delay inductor along with the motor inductance also serve to filter the output voltage and current from the chopper. The higher the chopper frequency, the less filtering required. An average d-c bus voltage will thus be applied to the inverter and an average bus current will be supplied through the inverter to the motor, the average values being determined by the duty cyle of the chopper. The greater the duty cycle, the wider the voltage pulses and the greater the average bus voltage and average bus current. A free-wheeling diode, which is considered to be part of the chopper, is shunt-connected across the d-c bus to provide a path for the reactive energy in the delay and motor inductances each time the chopper switch is turned off. When the voltage applied to an inductance is suddenly terminated, the inductance tends to keep the current flowing in the same direction thereby smoothing out the bus current. Hence, with this arrangement the motor current continues to flow through the same circuit and in the same direction during the off times of the chopper switch. As long as the chopper switch is being alternated between open and closed positions, continuous d-c bus current will flow to the motor.
The problem encountered in trying to start a chopper energized motor-commutated inverter is that the reactive energy cannot be regenerated back through the rectifier bridge, which comprises diode rectifiers rather than controllable SCR's, to the a-c line for rapid absorption as is possible when a phase-controlled rectifier bridge is employed. When a chopper is operated to develop a current pulse for energizing a motor and turning it one step, the reactive energy in the delay and motor inductances causes motor current to continue to flow through the previously conducting thyristors and the free-wheeling diode until the energy is eventually dissipated and the current decays to zero. Unfortunately, with such a discharge a relatively long time would be required to completely dissipate the reactive energy and reduce the motor current to zero. Since another energizing current pulse cannot be produced by the chopper and a new set of thyristors gated on until the current flowing through the previously conducting thyristors decays to zero, the relatively long decay time limits the rate at which the thyristors may be gated on during the starting mode. Actually, the decay time may be so long that the motor cannot be stepped around fast enough for a sufficient back EMF to develop to start motor commutating.
This starting problem of chopper controlled motor-commutated thyristor inverters has now been overcome by the present invention. A unique starting system of relatively simple and inexpensive construction is provided for efficiently and rapidly starting any chopper powered motor-commutated inverter.